Physicists don’t even know why it started expanding to begin with. We also don’t know if there’s anything outside of our own universe. We also don’t know if our universe is curved and folded in on itself, which would make several mathematical calculations for the size of the universe and what was going on with expansion a bit easier to try and work out (I’m also not a physicist. These are just things ive read about) or if it’s flat. Their best measurements right now is that it’s flat. But they still aren’t sure, because they don’t know how big space actually is right now. If it’s big enough, it could still be curved in on itself, but we just can’t measure the flatness of two points far enough apart from each other to notice the curve. An example I given was that it would be like trying to show the earth was round by measuring an area of a sandbox.